Electrostatic coating techniques involve electrically depositing atomized paint onto a piece to be coated (workpiece) by means of electrostatic force, and rotary electrostatic coating devices provided with a rotary head are known as devices for achieving this, wherein a typical example of the rotary head is a cup-shaped bell cup. Electrostatic coating devices of this type are employed for powdered paint, insulating liquid paint (e.g. oil-based paint), and conductive liquid paint (e.g. metallic paint or water-based paint), and there are also rotary electrostatic coating devices which are known of the type in which a high voltage is applied to the rotary head, and of the type in which a high voltage is applied to an external electrode which is remote from the rotary head in the outwardly radial direction.
Rotary electrostatic coating devices employ shaping air in order to direct paint onto the piece to be coated, and the coating pattern is dictated by means of this shaping air. As disclosed in the prior art section of Patent Document 1, the shaping air flows out from shaping air holes which are positioned to the rear of the bell cup, and it is then directed to the outer peripheral edge of the bell cup, and this has been the practice in the past. The shaping air which flows out from the shaping air holes strikes the outer peripheral edge of the back face of the bell cup, and therefore the flow speed thereof is reduced. This means that the flow of shaping air which has already passed by the bell cup is drawn radially inward because of the negative pressure in front of the bell cup which is produced by the flow of shaping air, and as a result the diameter of the coating pattern tends to be reduced.
It is better for the minute paint particles in metallic paint to strike the surface of a workpiece at high speed, something which is known in the art. However, as the flow speed of the shaping air increases, so the negative pressure in front of the bell cup increases, as a result of which there are problems in that the diameter of the coating pattern becomes smaller. The response to this problem in Patent Document 1 concerns the orientation of the shaping air which flows out from the shaping air holes, and that document proposes setting the orientation of the shaping air holes such that the torsion direction thereof is oriented about the axis of rotation of the bell cup. The shaping air forms a helical swirling flow due to the shaping air holes whereof the torsion direction has been oriented in this way, and the diameter of the coating pattern can be increased by the centrifugal force of this swirling flow.
Patent Document 2 proposes an improvement on the inventions of Patent Document 1. That is to say, the inventions of Patent Document 1 resolve the problems when the flow speed of the shaping air is increased for metallic coating, but Patent Document 2 focuses on problems such as excess spraying leading to paint loss when the diameter of the coating pattern is constant, for example when a narrow area such as an automobile pillar is being coated, and proposes an idea to improve on this situation.
The inventions proposed in Patent Document 2 are based on setting the orientation of the shaping air, which was proposed in Patent Document 1, in other words on setting the orientation of the shaping air holes in a torsion direction about the axis of rotation of the bell cup, and in said document, control air holes are provided further outward in the radial direction than the shaping air holes, and pattern control air which flows out from these control air holes strikes the shaping air at the outer peripheral edge of the bell cup, then the amount of outflow of pattern control air is changed, whereby the coating pattern width is controlled. In this instance, the control air holes which are positioned radially outward of the shaping air holes have a zero torsion angle about the axis of rotation of the bell cup, and they are inclined toward the axis of rotation of the bell cup. In other words, the shaping air holes have a torsion angle about the axis of rotation of the bell cup, and they are directed at the outer peripheral edge of the back face of the bell cup. In contrast to this, the control air holes have a zero torsion angle about the axis of rotation of the bell cup. The control air is inclined toward the axis of rotation of the bell cup, and therefore it merges with the shaping air at the outer peripheral edge of the bell cup.
When the outflow of pattern control air is stopped, the centrifugal force of the shaping air which swirls helically overcomes the suction force due to the negative pressure in front of the bell cup, whereby a coating pattern of relatively large diameter is formed. On the other hand, when the pattern control air is made to flow out, this pattern control air has a zero torsion angle, and therefore the pattern control air merges with the shaping air, whereby the torsion angle of the shaping air is substantially reduced, as a result of which the swirling force of the shaping air which swirls helically is weakened. Accordingly, the centrifugal force of the shaping air is relatively small, and therefore the effect of the negative pressure in front of the bell cup is weakened, and the diameter of the coating pattern is reduced.
As described above, Patent Document 2 relates to metallic coating, and it proposes reducing the diameter of the coating pattern by reducing the torsion angle of the shaping airflow which swirls helically, using control air which merges with the shaping air.
Patent Document 3 offers another proposal relating to variable control of the diameter of the coating pattern. The proposal of Patent Document 3 is similar to Patent Document 2 in that control air holes are provided radially outward of the shaping air holes, but the inventions of Patent Document 3 differ from those of Patent Document 2 firstly in that the control air is made to swirl helically, and they differ from Patent Document 2 secondly in that the pattern control air is parallel to the flow of shaping air (the control air and the shaping air do not merge).
To be more specific, in the inventions of Patent Document 3, a flow of pattern control air which swirls helically at a torsion angle which is different from that of the shaping air is generated radially to the outer periphery of the flow of shaping air which swirls helically, and then the amount of outflow of this pattern control air is controlled to cause the general torsion angle of the shaping air to change. Changing the diameter of the coating pattern by changing the general torsion angle of the shaping air is as described for Patent Document 2.    Patent Document 1: Japanese Unexamined Patent Application Publication H3-101858    Patent Document 2: Japanese Unexamined Patent Application Publication H7-24367    Patent Document 3: Japanese Unexamined Patent Application Publication H8-84941